Petrocodon rubiginosus, a new species of Gesneriaceae from Guangxi, China

Abstract Petrocodon rubiginosus Y.G. Wei & R.L. Zhang, sp. nov., from Guangxi of South China, is described and illustrated with photographs. The new species is morphologically similar to Pet. hechiensis, but can be easily distinguished by a combination of characters, especially in its petioles, peduncles and pedicels covered with densely ferruginous pilose hairs.


Introduction
Although the newly delimitation Petrocodon Hance is not the most speciose genus in Gesneriaceae of China, the highly variable corolla and leaf morphology impels us to continuously study and understand the species biodiversity of this genus (Lu et al. 2017. Now, approximately 35 species and one variety range from China to northern Thailand and northern Vietnam , Chen et al. 2014, Xu et al. 2014, Middleton et al. 2015, 2013, 2016. During a floristic expedition to Guangxi, China in 2015, the authors observed a population of an interesting Gesneriaceae in Yuexu Town, Jingxi city, Guangxi. We confirmed that it is a member of the genus Petrocodon because it looks like Pet. hechiensis (Y.G.Wei, Yan Liu & F.Wen) Y.G. Wei & Mich.Möller (Wei et al. 2008. Over the past three years, the living plants were monitored in the conservation nursery of the Gesneriad Conservation Centre of China (GCCC) in Guilin Botanical Garden and in the field, where an ecological survey was conducted.
After thorough comparisons of diagnostic morphological and anatomical features of similar taxa from China, Vietnam and Thailand (Wang et al. 1990, 1998, Li and Wang 2005, Phuong 2005, Wei et al. 2010, Middleton et al. 2015, it has been revealed that its morphological characters do not fit any known species, therefore, we conclude that it is a new species to science and accordingly describe it herein. Its morphological characters are compared with the closely related species, Petrocodon hechiensis.

Material and methods
Measurements and morphological character assessments of the new species were performed and described using specimens obtained by the current authors, living material observed in the field and also cultivated at GCCC. All available Petrocodon specimens of China, Thailand and Vietnam, stored in the following herbaria, were examined: E, GH, HN, IBK, K, KUN, MO, PE, PH, US and VNMN. At the same time, the specimen's images and name list of the above-mentioned species were obtained and checked from Tropicos (http://www.tropicos.org), JSTOR Global Plants (http:// plants.jstor.org), The Plant List (http://www.plantlist.org/) and the International Plant Names Index (http://www.ipni.org). All morphological characters were studied under dissecting microscopes and are described using the terminology presented by Wang et al. (1990Wang et al. ( , 1998.

Taxonomic Treatment
Petrocodon rubiginosus Y.G.Wei & R.L.Zhang, sp. nov. urn:lsid:ipni.org:names:77211194-1 Figure 1 Diagnosis. It is morphologically close to Pet. hechiensis, but can be distinguished by its petioles, peduncles and pedicels densely curly rubiginous to ferruginous villous, bracts lanceolate to subulate and both surfaces densely rubiginous to dark brown pubescent, corolla lobes oblong to nearly rounded and margin entire, calyx lobes densely rubiginous to ferruginous pubescent.
Phenology. Flowering occurs in November and fruiting from December to January of next year.
Distribution and habitat. Petrocodon rubiginosus is hitherto only known from two close localities at elevational ranges from 450-500 m in Jingxi city, Guangxi, China, growing on shaded and moist rock surface with no more than 50 individuals at each of the localities. Two localities are about three kilometres apart. The species grows in subtropical broad-leaved evergreen monsoon forest with sufficient seasonal run-off water.
Preliminary conservation assessment. Population information of Petrocodon rubiginosus is still unclear, which makes it difficult to determine an assessment of the extinction risk faced by this new taxon. At present, two distribution points of the species are known and its estimated area of occupancy is less than 10 km 2 . The main threat now comes from environmental damage caused by grazing and there is a risk of poaching in the future because its distribution is not far away from the villages. Furthermore, prolonged droughts and illegal logging in the area, including nearby potential habitat, should be considered as potential risks to the persistence of Pet. rubiginosus. Thus, following the IUCN Red List Categories and Criteria (IUCN 2017), it is assessed temporarily as endangered [EN B2ab (ii,iii)].
Notes. It is morphologically close to Petrocodon hechiensis because two congeners share some similarities, for example, they have look-alike leaf shape and size and a pale-yellow corolla (Wei et al. 2008(Wei et al. , 2010. As a result, it is sometimes mistaken for Pet. hechiensis. However, one of the largest differences between Pet. rubiginosus and Pet. hechiensis is the indumentum of petioles, peduncles and pedicels. There are absolutely no glandular-hairs on petioles, peduncles and pedicels of Pet. rubiginosus, but in Pet. hechiensis, all are covered by densely short and sticky glandular-hairs and pubescenthairs (Figure 3). The other major differences between the species are outlined in Table 1.  Table 1. Diagnostic character differences between Petrocodon rubiginosus sp. nov. and Pet. hechiensis (Figure 2).